Solid electrolytic capacitor with an ultrahigh capacitance

ABSTRACT

A solid electrolytic capacitor that comprises an anode that contains a dielectric formed on a sintered porous body is provided. The sintered porous body is formed from a valve metal powder having a specific charge of about 100,000 microFarads*Volts per gram or more. The solid electrolyte overlies the anode, and includes an intrinsically conductive polymer containing repeating units having the following formula (I): 
                         
wherein,
         R is (CH 2 ) a —O—(CH 2 ) b ;   a is from 0 to 10;   b is from 1 to 18;   Z is an anion; and   X is a cation.

BACKGROUND OF THE INVENTION

Solid electrolytic capacitors (e.g., tantalum capacitors) are typicallymade by pressing a metal powder (e.g., tantalum) around a metal leadwire, sintering the pressed part, anodizing the sintered anode, andthereafter applying a solid electrolyte. Intrinsically conductivepolymers are often employed as the solid electrolyte due to theiradvantageous low equivalent series resistance (“ESR”) and“non-burning/non-ignition” failure mode. Such electrolytes can be formedthrough in situ chemical polymerization of the monomer in the presenceof a catalyst and dopant. One of the problems with conventionalcapacitors that employ in situ polymerized polymers is that they tend tofail at high voltages, such as experienced during a fast switch on oroperational current spike. In an attempt to overcome some of theseissues, premade conductive polymer slurries have also been employed incertain applications as an alternative solid electrolyte material. Whilesome benefits have been achieved with these capacitors, problemsnevertheless remain. For many applications, it is often desirable to usemetal powders having an ultrahigh specific charge—i.e., about 100,000microFarads*Volts per gram (μF*V/g″) or more. Such ultrahigh “CV/g”powders are generally formed from particles having a nano-scale size,which results in the formation of very small pores between theparticles. Unfortunately, it is often difficult to impregnate premadepolymer slurries into these small pores, which has traditionally led torelatively poor electrical performance of the capacitor. Another problemwith polymer slurry-based capacitors is that they can achieve only arelatively low percentage of their wet capacitance, which means thatthey have a relatively large capacitance loss and/or fluctuation in thepresence of atmosphere humidity.

As such, a need currently exists for a solid electrolytic capacitorhaving an improved performance.

SUMMARY OF THE INVENTION

In accordance with one embodiment of the present invention, a solidelectrolytic capacitor is disclosed that comprises an anode thatcontains a dielectric formed on a sintered porous body, wherein thesintered porous body is formed from a valve metal powder having aspecific charge of about 100,000 microFarads*Volts per gram or more. Thesolid electrolyte overlies the anode, and includes an intrinsicallyconductive polymer containing repeating units having the followingformula (I):

wherein,

R is (CH₂)_(a)—O—(CH₂)_(b);

a is from 0 to 10;

b is from 1 to 18;

Z is an anion; and

X is a cation.

In accordance with another embodiment of the present invention, a methodfor forming a solid electrolytic capacitor is disclosed that comprisespressing a powder into the form of a pellet, wherein the powder has aspecific charge of about 100,000 μF*V/g or more; sintering the pellet;anodically oxidizing the sintered pellet to form a dielectric layer thatoverlies the anode; and dipping the anodically oxidized pellet into asolution that contains an intrinsically conductive polymer containingrepeating units having the following formula (I):

wherein,

R is (CH₂)_(a)—O—(CH₂)_(b);

a is from 0 to 10;

b is from 1 to 18;

Z is an anion; and

X is a cation.

Other features and aspects of the present invention are set forth ingreater detail below.

BRIEF DESCRIPTION OF THE DRAWING

A full and enabling disclosure of the present invention, including thebest mode thereof, directed to one of ordinary skill in the art, is setforth more particularly in the remainder of the specification, whichmakes reference to the appended FIGURE in which:

FIG. 1 is a schematic illustration of one embodiment of a capacitor thatmay be formed in accordance with the present invention.

Repeat use of references characters in the present specification anddrawing is intended to represent same or analogous features or elementsof the invention.

DETAILED DESCRIPTION OF REPRESENTATIVE EMBODIMENTS

It is to be understood by one of ordinary skill in the art that thepresent discussion is a description of exemplary embodiments only, andis not intended as limiting the broader aspects of the presentinvention, which broader aspects are embodied in the exemplaryconstruction.

Generally speaking, the present invention is directed to a solidelectrolytic capacitor that includes an anode that contains a dielectricformed on a sintered porous body, and a solid electrolyte overlying theanode. The sintered porous body is formed from a valve metal powderhaving an ultrahigh specific charge. The specific charge of the powdermay, for instance be about 100,000 microFarads*Volts per gram (“μF*V/g”)or more, in some embodiments from about 120,000 to about 800,000 μF*V/g,and in some embodiments, from about 150,000 to about 600,000 μf*V/g. Asis known in the art, the specific charge may be determined bymultiplying capacitance by the anodizing voltage employed, and thendividing this product by the weight of the anodized electrode body.Despite being formed from a powder having an ultrahigh specific charge,the present inventors have nevertheless discovered that a capacitorhaving excellent electrical properties can still be formed throughselective control over the solid electrolyte. That is, the solidelectrolyte includes a polymer that is considered “intrinsically”conductive to the extent that it has a positive charge located on themain chain that is at least partially compensated by anions covalentlybound to the polymer. More particularly, the polymer contains repeatingunits having the following formula (I):

wherein,

R is (CH₂)_(a)—O—(CH₂)_(b);

a is from 0 to 10, in some embodiments from 0 to 6, and in someembodiments, from 1 to 4 (e.g., 1);

b is from 1 to 18, in some embodiments from 1 to 10, and in someembodiments, from 2 to 6 (e.g., 3, 4, or 5);

Z is an anion, such as SO₃ ⁻, C(O)O⁻, BF₄ ⁻, CF₃SO₃ ⁻, SbF₆ ⁻,N(SO₂CF₃)₂ ⁻, C₄H₃O₄ ⁻, ClO₄ ⁻, etc.;

X is a cation, such as hydrogen, an alkali metal (e.g., lithium, sodium,rubidium, cesium or potassium), ammonium, etc.

The polymer may, for example, have a relatively high specificconductivity, in the dry state, of about 1 Siemen per centimeter(“S/cm”) or more, in some embodiments about 10 S/cm or more, in someembodiments about 20 S/cm or more, and in some embodiments, from about50 to about 500 S/cm. As a result of its intrinsic conductivity, thelayer does not require the addition of conventional dopants, such aspolystyrene sulfonic acid. In fact, the layer may be substantially freeof such dopants. Nevertheless, it should be understood that dopants maybe employed in certain embodiments of the present invention. Whenutilized, however, dopants are typically present in the layer in anamount of about 5 wt. % or less, in some embodiments about 2 wt. % orless, and in some embodiments, about 1 wt. % or less.

The intrinsically conductive polymers can also have a variety of otherbenefits. For example, the polymer is generally highly soluble in water,which enables it to be more easily and effectively applied to the anode.The soluble polymer is also able to more readily impregnate the smallpores formed by the high specific charge powder, so that the resultingsolid electrolyte has a “film-like” configuration and coats at least aportion of the anode in a substantially uniform manner. This improvesthe quality of the resulting oxide as well as its surface coverage, andthereby enhances the electrical properties of the capacitor. Forexample, the resulting capacitor can exhibit a high percentage of itswet capacitance, which enables it to have only a small capacitance lossand/or fluctuation in the presence of atmosphere humidity. Thisperformance characteristic is quantified by the “wet-to-dry capacitancepercentage”, which is determined by the equation:Wet-to-Dry Capacitance=(Dry Capacitance/Wet Capacitance)×100

The capacitor of the present invention may exhibit a wet-to-drycapacitance percentage of about 50% or more, in some embodiments about60% or more, in some embodiments about 70% or more, and in someembodiments, from about 80% to 100%. The dry capacitance may be about 30nanoFarads per square centimeter (“nF/cm²”) or more, in some embodimentsabout 100 nF/cm² or more, in some embodiments from about 200 to about3,000 nF/cm², and in some embodiments, from about 400 to about 2,000nF/cm², measured at a frequency of 120 Hz.

The capacitor may also exhibit a relatively low equivalence seriesresistance (“ESR”), such as about 200 mohms, in some embodiments lessthan about 150 mohms, and in some embodiments, from about 0.1 to about100 mohms, measured at an operating frequency of 100 kHz. It is alsobelieved that the dissipation factor of the capacitor may be maintainedat relatively low levels. The dissipation factor generally refers tolosses that occur in the capacitor and is usually expressed as apercentage of the ideal capacitor performance. For example, thedissipation factor of the capacitor of the present invention istypically from about 1% to about 25%, in some embodiments from about 3%to about 10%, and in some embodiments, from about 5% to about 15%, asdetermined at a frequency of 120 Hz. The capacitor may also be able tobe employed in high voltage applications, such as at rated voltages ofabout 35 volts or more, in some embodiments about 50 volts or more, andin some embodiments, from about 60 volts to about 200 volts. Thecapacitor may, for example, exhibit a relatively high “breakdownvoltage” (voltage at which the capacitor fails), such as about 2 voltsor more, in some embodiments about 5 volts or more, in some embodimentsabout 10 volts or more, and in some embodiments, from about 10 to about100 volts. Likewise, the capacitor may also be able to withstandrelatively high surge currents, which is also common in high voltageapplications. The peak surge current may be, for example, about 100 Ampsor more, in some embodiments about 200 Amps or more, and in someembodiments, and in some embodiments, from about 300 Amps to about 800Amps.

Various embodiments of the present invention will now be described inmore detail.

I. Anode Body

As noted, the porous anode body is formed from a powder that contains avalve metal (i.e., metal that is capable of oxidation) or valvemetal-based compound, such as tantalum, niobium, aluminum, hafnium,titanium, alloys thereof, oxides thereof, nitrides thereof, and soforth. The powder is typically formed from a reduction process in whicha tantalum salt (e.g., potassium fluotantalate (K₂TaF₇), sodiumfluotantalate (Na₂TaF₇), tantalum pentachloride (TaCl₅), etc.) isreacted with a reducing agent. The reducing agent may be provided in theform of a liquid, gas (e.g., hydrogen), or solid, such as a metal (e.g.,sodium), metal alloy, or metal salt. In one embodiment, for instance, atantalum salt (e.g., TaCl₅) may be heated at a temperature of from about900° C. to about 2,000° C., in some embodiments from about 1,000° C. toabout 1,800° C., and in some embodiments, from about 1,100° C. to about1,600° C., to form a vapor that can be reduced in the presence of agaseous reducing agent (e.g., hydrogen). Additional details of such areduction reaction may be described in WO 2014/199480 to Maeshima, etal. After the reduction, the product may be cooled, crushed, and washedto form a powder.

The powder may be a free-flowing, finely divided powder that containsprimary particles. The primary particles of the powder generally have amedian size (D50) of from about 5 to about 500 nanometers, in someembodiments from about 10 to about 400 nanometers, and in someembodiments, from about 20 to about 250 nanometers, such as determinedusing a laser particle size distribution analyzer made by BECKMANCOULTER Corporation (e.g., LS-230), optionally after subjecting theparticles to an ultrasonic wave vibration of 70 seconds. The primaryparticles typically have a three-dimensional granular shape (e.g.,nodular or angular). Such particles typically have a relatively low“aspect ratio”, which is the average diameter or width of the particlesdivided by the average thickness (“D/T”). For example, the aspect ratioof the particles may be about 4 or less, in some embodiments about 3 orless, and in some embodiments, from about 1 to about 2. In addition toprimary particles, the powder may also contain other types of particles,such as secondary particles formed by aggregating (or agglomerating) theprimary particles. Such secondary particles may have a median size (D50)of from about 1 to about 500 micrometers, and in some embodiments, fromabout 10 to about 250 micrometers.

Agglomeration of the particles may occur by heating the particles and/orthrough the use of a binder. For example, agglomeration may occur at atemperature of from about 0° C. to about 40° C., in some embodimentsfrom about 5° C. to about 35° C., and in some embodiments, from about15° C. to about 30° C. Suitable binders may likewise include, forinstance, poly(vinyl butyral); poly(vinyl acetate); poly(vinyl alcohol);poly(vinyl pyrollidone); cellulosic polymers, such ascarboxymethylcellulose, methyl cellulose, ethyl cellulose, hydroxyethylcellulose, and methylhydroxyethyl cellulose; atactic polypropylene,polyethylene; polyethylene glycol (e.g., Carbowax from Dow ChemicalCo.); polystyrene, poly(butadiene/styrene); polyamides, polyimides, andpolyacrylamides, high molecular weight polyethers; copolymers ofethylene oxide and propylene oxide; fluoropolymers, such aspolytetrafluoroethylene, polyvinylidene fluoride, and fluoro-olefincopolymers; acrylic polymers, such as sodium polyacrylate, poly(loweralkyl acrylates), poly(lower alkyl methacrylates) and copolymers oflower alkyl acrylates and methacrylates; and fatty acids and waxes, suchas stearic and other soapy fatty acids, vegetable wax, microwaxes(purified paraffins), etc.

The resulting powder may be compacted to form a pellet using anyconventional powder press device. For example, a press mold may beemployed that is a single station compaction press containing a die andone or multiple punches. Alternatively, anvil-type compaction pressmolds may be used that use only a die and single lower punch. Singlestation compaction press molds are available in several basic types,such as cam, toggle/knuckle and eccentric/crank presses with varyingcapabilities, such as single action, double action, floating die,movable platen, opposed ram, screw, impact, hot pressing, coining orsizing. The powder may be compacted around an anode lead, which may bein the form of a wire, sheet, etc. The lead may extend in a longitudinaldirection from the anode body and may be formed from any electricallyconductive material, such as tantalum, niobium, aluminum, hafnium,titanium, etc., as well as electrically conductive oxides and/ornitrides of thereof. Connection of the lead may also be accomplishedusing other known techniques, such as by welding the lead to the body orembedding it within the anode body during formation (e.g., prior tocompaction and/or sintering).

Any binder may be removed after pressing by heating the pellet undervacuum at a certain temperature (e.g., from about 150° C. to about 500°C.) for several minutes. Alternatively, the binder may also be removedby contacting the pellet with an aqueous solution, such as described inU.S. Pat. No. 6,197,252 to Bishop, et al. Thereafter, the pellet issintered to form a porous, integral mass. The pellet is typicallysintered at a temperature of from about 700° C. to about 1600° C., insome embodiments from about 800° C. to about 1500° C., and in someembodiments, from about 900° C. to about 1200° C., for a time of fromabout 5 minutes to about 100 minutes, and in some embodiments, fromabout 8 minutes to about 15 minutes. This may occur in one or moresteps. If desired, sintering may occur in an atmosphere that limits thetransfer of oxygen atoms to the anode. For example, sintering may occurin a reducing atmosphere, such as in a vacuum, inert gas, hydrogen, etc.The reducing atmosphere may be at a pressure of from about 10 Torr toabout 2000 Torr, in some embodiments from about 100 Torr to about 1000Torr, and in some embodiments, from about 100 Torr to about 930 Torr.Mixtures of hydrogen and other gases (e.g., argon or nitrogen) may alsobe employed.

II. Dielectric

The anode is also coated with a dielectric. The dielectric may be formedby anodically oxidizing (“anodizing”) the sintered anode so that adielectric layer is formed over and/or within the anode. For example, atantalum (Ta) anode may be anodized to tantalum pentoxide (Ta₂O₅).Typically, anodization is performed by initially applying a solution tothe anode, such as by dipping anode into the electrolyte. A solvent isgenerally employed, such as water (e.g., deionized water). To enhanceionic conductivity, a compound may be employed that is capable ofdissociating in the solvent to form ions. Examples of such compoundsinclude, for instance, acids, such as described below with respect tothe electrolyte. For example, an acid (e.g., phosphoric acid) mayconstitute from about 0.01 wt. % to about 5 wt. %, in some embodimentsfrom about 0.05 wt. % to about 0.8 wt. %, and in some embodiments, fromabout 0.1 wt. % to about 0.5 wt. % of the anodizing solution. Ifdesired, blends of acids may also be employed.

A current is passed through the anodizing solution to form thedielectric layer. The value of the formation voltage manages thethickness of the dielectric layer. For example, the power supply may beinitially set up at a galvanostatic mode until the required voltage isreached. Thereafter, the power supply may be switched to apotentiostatic mode to ensure that the desired dielectric thickness isformed over the entire surface of the anode. Of course, other knownmethods may also be employed, such as pulse or step potentiostaticmethods. The voltage at which anodic oxidation occurs typically rangesfrom about 4 to about 250 V, and in some embodiments, from about 5 toabout 200 V, and in some embodiments, from about 10 to about 150 V.During oxidation, the anodizing solution can be kept at an elevatedtemperature, such as about 30° C. or more, in some embodiments fromabout 40° C. to about 200° C., and in some embodiments, from about 50°C. to about 100° C. Anodic oxidation can also be done at ambienttemperature or lower. The resulting dielectric layer may be formed on asurface of the anode and within its pores.

Although not required, in certain embodiments, the dielectric layer maypossess a differential thickness throughout the anode in that itpossesses a first portion that overlies an external surface of the anodeand a second portion that overlies an interior surface of the anode. Insuch embodiments, the first portion is selectively formed so that itsthickness is greater than that of the second portion. It should beunderstood, however, that the thickness of the dielectric layer need notbe uniform within a particular region. Certain portions of thedielectric layer adjacent to the external surface may, for example,actually be thinner than certain portions of the layer at the interiorsurface, and vice versa. Nevertheless, the dielectric layer may beformed such that at least a portion of the layer at the external surfacehas a greater thickness than at least a portion at the interior surface.Although the exact difference in these thicknesses may vary depending onthe particular application, the ratio of the thickness of the firstportion to the thickness of the second portion is typically from about1.2 to about 40, in some embodiments from about 1.5 to about 25, and insome embodiments, from about 2 to about 20.

To form a dielectric layer having a differential thickness, amulti-stage process is generally employed. In each stage of the process,the sintered anode is anodically oxidized (“anodized”) to form adielectric layer (e.g., tantalum pentoxide). During the first stage ofanodization, a relatively small forming voltage is typically employed toensure that the desired dielectric thickness is achieved for the innerregion, such as forming voltages ranging from about 1 to about 90 volts,in some embodiments from about 2 to about 50 volts, and in someembodiments, from about 5 to about 20 volts. Thereafter, the sinteredbody may then be anodically oxidized in a second stage of the process toincrease the thickness of the dielectric to the desired level. This isgenerally accomplished by anodizing in an electrolyte at a highervoltage than employed during the first stage, such as at formingvoltages ranging from about 50 to about 350 volts, in some embodimentsfrom about 60 to about 300 volts, and in some embodiments, from about 70to about 200 volts. During the first and/or second stages, theelectrolyte may be kept at a temperature within the range of from about15° C. to about 95° C., in some embodiments from about 20° C. to about90° C., and in some embodiments, from about 25° C. to about 85° C.

The electrolytes employed during the first and second stages of theanodization process may be the same or different. Typically, however, itis desired to employ different solutions to help better facilitate theattainment of a higher thickness at the outer portions of the dielectriclayer. For example, it may be desired that the electrolyte employed inthe second stage has a lower ionic conductivity than the electrolyteemployed in the first stage to prevent a significant amount of oxidefilm from forming on the internal surface of anode. In this regard, theelectrolyte employed during the first stage may contain an acidiccompound, such as hydrochloric acid, nitric acid, sulfuric acid,phosphoric acid, polyphosphoric acid, boric acid, boronic acid, etc.Such an electrolyte may have an electrical conductivity of from about0.1 to about 100 mS/cm, in some embodiments from about 0.2 to about 20mS/cm, and in some embodiments, from about 1 to about 10 mS/cm,determined at a temperature of 25° C. The electrolyte employed duringthe second stage typically contains a salt of a weak acid so that thehydronium ion concentration increases in the pores as a result of chargepassage therein. Ion transport or diffusion is such that the weak acidanion moves into the pores as necessary to balance the electricalcharges. As a result, the concentration of the principal conductingspecies (hydronium ion) is reduced in the establishment of equilibriumbetween the hydronium ion, acid anion, and undissociated acid, thusforms a poorer-conducting species. The reduction in the concentration ofthe conducting species results in a relatively high voltage drop in theelectrolyte, which hinders further anodization in the interior while athicker oxide layer is being built up on the outside to a higherformation voltage in the region of continued high conductivity. Suitableweak acid salts may include, for instance, ammonium or alkali metalsalts (e.g., sodium, potassium, etc.) of boric acid, boronic acid,acetic acid, oxalic acid, lactic acid, adipic acid, etc. Particularlysuitable salts include sodium tetraborate and ammonium pentaborate. Suchelectrolytes typically have an electrical conductivity of from about 0.1to about 20 mS/cm, in some embodiments from about 0.5 to about 10 mS/cm,and in some embodiments, from about 1 to about 5 mS/cm, determined at atemperature of 25° C.

If desired, each stage of anodization may be repeated for one or morecycles to achieve the desired dielectric thickness. Furthermore, theanode may also be rinsed or washed with another solvent (e.g., water)after the first and/or second stages to remove the electrolyte.

III. Solid Electrolyte

A solid electrolyte overlies the dielectric that generally functions asthe cathode for the capacitor. Typically, the total thickness of thesolid electrolyte is from about 1 to about 50 μm, and in someembodiments, from about 5 to about 20 μm. The solid electrolyte may beformed from one or multiple conductive polymer layers, at least one ofwhich includes an intrinsically conductive polymer having repeatingunits of the following formula (I):

wherein,

R is (CH₂)_(a)—O—(CH₂)_(b);

a is from 0 to 10, in some embodiments from 0 to 6, and in someembodiments, from 1 to 4 (e.g., 1);

b is from 1 to 18, in some embodiments from 1 to 10, and in someembodiments, from 2 to 6 (e.g., 2, 3, 4, or 5);

Z is an anion, such as SO₃ ⁻, C(O)O⁻, BF₄ ⁻, CF₃SO₃ ⁻, SbF₆ ⁻,N(SO₂CF₃)₂ ⁻, C₄H₃O₄ ⁻, ClO₄ ⁻, etc.;

X is a cation, such as hydrogen, an alkali metal (e.g., lithium, sodium,rubidium, cesium or potassium), ammonium, etc.

In one particular embodiment, Z in formula (I) is a sulfonate ion suchthat the intrinsically conductive polymer contains repeating units ofthe following formula (II):

wherein, R and X are defined above. In formula (I) or (II), a ispreferably 1 and b is preferably 3 or 4. Likewise, X is preferablysodium or potassium.

If desired, the polymer may be a copolymer that contains other types ofrepeating units. In such embodiments, the repeating units of formula (I)typically constitute about 50 mol. % or more, in some embodiments fromabout 75 mol. % to about 99 mol. %, and in some embodiments, from about85 mol. % to about 95 mol. % of the total amount of repeating units inthe copolymer. Of course, the polymer may also be a homopolymer to theextent that it contains 100 mol. % of the repeating units of formula(I). Specific examples of such homopolymers includepoly(4-(2,3-dihydrothieno-[3,4-b][1,4]dioxin-2-ylmethoxy)-1-butane-sulphonicacid, salt) andpoly(4-(2,3-dihydrothieno-[3,4-b][1,4]dioxin-2-ylmethoxy)-1-propanesulphonicacid, salt).

The intrinsically conductive polymer may be formed through a variety oftechniques as would be understood by those skilled in the art. In oneparticular embodiment, for example, a 3,4-ethylenedioxythiophene saltmay be polymerized in the presence of an oxidative catalyst. Derivativesof these monomers may also be employed that are, for example, dimers orturners of the above compounds. The derivatives may be made up ofidentical or different monomer units and used in pure form and in amixture with one another and/or with the monomers. Oxidized or reducedforms of these precursors may also be employed. The oxidative catalystmay be a transition metal salt, such as a salt of an inorganic ororganic acid that contain ammonium, sodium, gold, iron(III), copper(II),chromium(VI), cerium(IV), manganese(IV), manganese(VII), orruthenium(III) cations. Particularly suitable transition metal saltsinclude halides (e.g., FeCl₃ or HAuCl₄); salts of other inorganic acids(e.g., Fe(ClO₄)₃, Fe₂(SO₄)₃, (NH₄)₂S₂O₈, or Na₃Mo₁₂PO₄₀); and salts oforganic acids and inorganic acids comprising organic radicals. Examplesof salts of inorganic acids with organic radicals include, for instance,iron(III) salts of sulfuric acid monoesters of C₁ to C₂₀ alkanols (e.g.,iron(III) salt of lauryl sulfate). Likewise, examples of salts oforganic acids include, for instance, iron(III) salts of C₁ to C₂₀ alkanesulfonic acids (e.g., methane, ethane, propane, butane, or dodecanesulfonic acid); iron (III) salts of aliphatic perfluorosulfonic acids(e.g., trifluoromethane sulfonic acid, perfluorobutane sulfonic acid, orperfluorooctane sulfonic acid); iron (III) salts of aliphatic C₁ to C₂₀carboxylic acids (e.g., 2-ethylhexylcarboxylic acid); iron (III) saltsof aliphatic perfluorocarboxylic acids (e.g., trifluoroacetic acid orperfluorooctane acid); iron (III) salts of aromatic sulfonic acidsoptionally substituted by C₁ to C₂₀ alkyl groups (e.g., benzene sulfonicacid, o-toluene sulfonic acid, p-toluene sulfonic acid, ordodecylbenzene sulfonic acid); iron (III) salts of cycloalkane sulfonicacids (e.g., camphor sulfonic acid); and so forth. Mixtures of theseabove-mentioned salts may also be used.

Oxidative polymerization generally occurs in the presence of one or moresolvents. Suitable solvents may include, for instance, water, glycols(e.g., ethylene glycol, propylene glycol, butylene glycol, triethyleneglycol, hexylene glycol, polyethylene glycols, ethoxydiglycol,dipropyleneglycol, etc.); glycol ethers (e.g., methyl glycol ether,ethyl glycol ether, isopropyl glycol ether, etc.); alcohols (e.g.,methanol, ethanol, n-propanol, iso-propanol, and butanol); ketones(e.g., acetone, methyl ethyl ketone, and methyl isobutyl ketone); esters(e.g., ethyl acetate, butyl acetate, diethylene glycol ether acetate,methoxypropyl acetate, ethylene carbonate, propylene carbonate, etc.);amides (e.g., dimethylformamide, dimethylacetamide,dimethylcaprylic/capric fatty acid amide and N-alkylpyrrolidones);sulfoxides or sulfones (e.g., dimethyl sulfoxide (DMSO) and sulfolane);phenolic compounds (e.g., toluene, xylene, etc.), and so forth. Water isa particularly suitable solvent for the reaction. The temperature atwhich the reaction occurs typically varies from about −20° C. to about140° C., and in some embodiments, from about 20° C. to about 100° C.Upon completion of the reaction, known filtration techniques may beemployed to remove any salt impurities.

If desired, the conductive polymer layer may be generally free from“extrinsically” conductive polymers and thus formed entirely fromintrinsically conductive polymers. In such embodiments, the layer may beapplied in the form of a solution or dispersion containing a solvent,although a solution is typically preferred. The concentration of thepolymer may vary depending on the desired viscosity of and theparticular manner in which the layer is to be applied to the anode.Typically, however, the polymer constitutes from about 0.1 to about 10wt. %, in some embodiments from about 0.4 to about 5 wt. %, and in someembodiments, from about 0.5 to about 4 wt. % of the solution ordispersion. Solvent(s) may likewise constitute from about 90 wt. % toabout 99.9 wt. %, in some embodiments from about 95 wt. % to about 99.6wt. %, and in some embodiments, from about 96 wt. % to about 99.5 wt. %of the solution or dispersion. While other solvents may certainly beemployed, it is generally desired that water is the primary solvent suchthat the dispersion is considered an “aqueous” solution. In mostembodiments, for example, water constitutes at least about 50 wt. %, insome embodiments at least about 75 wt. %, and in some embodiments, fromabout 90 wt. % to 100 wt. % of the solvent(s) employed. When employed, asolution may be applied to the anode using any known technique, such asdipping, casting (e.g., curtain coating, spin coating, etc.), printing(e.g., gravure printing, offset printing, screen printing, etc.), and soforth. The resulting conductive polymer layer may be dried and/or washedafter it is applied to the anode. In alternative embodiments, otherpolymers may also be employed in combination with the intrinsicallyconductive polymer in the conductive polymer layer. For example, anotherconductive polymer may be employed in the solid electrolyte, such as apolypyrrole, polythiophene, polyaniline, polyacetylene,poly-p-phenylene, polyphenolate, and so forth. In one particularembodiment, the conductive polymer layer may also include a polymerhaving repeating units of the following formula (III):

wherein,

R₇ is a linear or branched, C₁ to C₁₈ alkyl radical (e.g., methyl,ethyl, n- or iso-propyl, n-, iso-, sec- or tert-butyl, n-pentyl,1-methylbutyl, 2-methylbutyl, 3-methylbutyl, 1-ethylpropyl,1,1-dimethylpropyl, 1,2-dimethylpropyl, 2,2-dimethylpropyl, n-hexyl,n-heptyl, n-octyl, 2-ethylhexyl, n-nonyl, n-decyl, n-undecyl, n-dodecyl,n-tridecyl, n-tetradecyl, n-hexadecyl, n-octadecyl, etc.); C₅ to C₁₂cycloalkyl radical (e.g., cyclopentyl, cyclohexyl, cycloheptyl,cyclooctyl, cyclononyl, cyclodecyl, etc.); C₆ to C₁₄ aryl radical (e.g.,phenyl, naphthyl, etc.); C₇ to C₁₈ aralkyl radical (e.g., benzyl, o-,m-, p-tolyl, 2,3-, 2,4-, 2,5-, 2-6, 3-4-, 3,5-xylyl, mesityl, etc.); C₁to C₄ hydroxyalkyl radical, or hydroxyl radical; and

q is an integer from 0 to 8, in some embodiments, from 0 to 2, and inone embodiment, 0. In one particular embodiment, “q” is 0 and thepolymer is poly(3,4-ethylenedioxythiophene). One commercially suitableexample of a monomer suitable for forming such a polymer is3,4-ethylenedioxthiophene, which is available from Heraeus under thedesignation Clevios™ M.

The polymers of formula (III) are generally considered to be“extrinsically” conductive to the extent that they require the presenceof a separate counterion that is not covalently bound to the polymer.The counterion may be a monomeric or polymeric anion that counteractsthe charge of the conductive polymer. Polymeric anions can, for example,be anions of polymeric carboxylic acids (e.g., polyacrytic acids,polymethacrylic acid, polymaleic acids, etc.); polymeric sulfonic acids(e.g., polystyrene sulfonic acids (“PSS”), polyvinyl sulfonic acids,etc.); and so forth. The acids may also be copolymers, such ascopolymers of vinyl carboxylic and vinyl sulfonic acids with otherpolymerizable monomers, such as acrylic acid esters and styrene.Likewise, suitable monomeric anions include, for example, anions of C₁to C₂₀ alkane sulfonic acids (e.g., dodecane sulfonic acid); aliphaticperfluorosulfonic acids (e.g., trifluoromethane sulfonic acid,perfluorobutane sulfonic acid or perfluorooctane sulfonic acid);aliphatic C₁ to C₂₀ carboxylic acids (e.g., 2-ethyl-hexylcarboxylicacid); aliphatic perfluorocarboxylic acids (e.g., trifluoroacetic acidor perfluorooctanoic acid); aromatic sulfonic acids optionallysubstituted by C₁ to C₂₀ alkyl groups (e.g., benzene sulfonic acid,o-toluene sulfonic acid, p-toluene sulfonic acid or dodecylbenzenesulfonic acid); cycloalkane sulfonic acids (e.g., camphor sulfonic acidor tetrafluoroborates, hexafluorophosphates, perchlorates,hexafluoroantimonates, hexafluoroarsenates or hexachloroantimonates);and so forth. Particularly suitable counteranions are polymeric anions,such as a polymeric carboxylic or sulfonic acid (e.g., polystyrenesulfonic acid (“PSS”)). The molecular weight of such polymeric anionstypically ranges from about 1,000 to about 2,000,000, and in someembodiments, from about 2,000 to about 500,000.

When such an extrinsically conductive polymer is employed, theconductive polymer layer may be applied in the form of a dispersion. Insuch embodiments, the conductive polymer may be in the form of particleshaving an average size (e.g., diameter) of from about 1 to about 150nanometers, in some embodiments from about 2 to about 50 nanometers, andin some embodiments, from about 5 to about 40 nanometers. The diameterof the particles may be determined using known techniques, such as byultracentrifuge, laser diffraction, etc. The shape of the particles maylikewise vary. In one particular embodiment, for instance, the particlesare spherical in shape. However, it should be understood that othershapes are also contemplated by the present invention, such as plates,rods, discs, bars, tubes, irregular shapes, etc. The concentration ofthe particles in the dispersion may vary depending on the desiredviscosity of the dispersion and the particular manner in which thedispersion is to be applied to the capacitor. Typically, however, theparticles constitute from about 0.1 to about 10 wt. %, in someembodiments from about 0.4 to about 5 wt. %, and in some embodiments,from about 0.5 to about 4 wt. % of the dispersion.

The dispersion may also contain one or more binders to further enhancethe adhesive nature of the polymeric layer and also increase thestability of the particles within the dispersion. The binders may beorganic in nature, such as polyvinyl alcohols, polyvinyl pyrrolidones,polyvinyl chlorides, polyvinyl acetates, polyvinyl butyrates,polyacrylic acid esters, polyacrylic acid amides, polymethacrylic acidesters, polymethacrylic acid amides, polyacrylonitriles, styrene/acrylicacid ester, vinyl acetate/acrylic acid ester and ethylene/vinyl acetatecopolymers, polybutadienes, polyisoprenes, polystyrenes, polyethers,polyesters, polycarbonates, polyurethanes, polyamides, polyimides,polysulfones, melamine formaldehyde resins, epoxide resins, siliconeresins or celluloses. Crosslinking agents may also be employed toenhance the adhesion capacity of the binders. Such crosslinking agentsmay include, for instance, melamine compounds, masked isocyanates orfunctional silanes, such as 3-glycidoxypropyltrialkoxysilane,tetraethoxysilane and tetraethoxysilane hydrolysate or crosslinkablepolymers, such as polyurethanes, polyacrylates or polyolefins, andsubsequent crosslinking.

Dispersion agents may also be employed to facilitate the ability toapply the layer to the anode. Suitable dispersion agents includesolvents, such as aliphatic alcohols (e.g., methanol, ethanol,i-propanol and butanol), aliphatic ketones (e.g., acetone and methylethyl ketones), aliphatic carboxylic acid esters (e.g., ethyl acetateand butyl acetate), aromatic hydrocarbons (e.g., toluene and xylene),aliphatic hydrocarbons (e.g., hexane, heptane and cyclohexane),chlorinated hydrocarbons (e.g., dichloromethane and dichloroethane),aliphatic nitriles (e.g., acetonitrile), aliphatic sulfoxides andsulfones (e.g., dimethyl sulfoxide and sulfolane), aliphatic carboxylicacid amides (e.g., methylacetamide, dimethylacetamide anddimethylformamide), aliphatic and araliphatic ethers (e.g., diethyletherand anisole), water, and mixtures of any of the foregoing solvents. Aparticularly suitable dispersion agent is water.

In addition to those mentioned above, still other ingredients may alsobe used in the dispersion. For example, conventional fillers may be usedthat have a size of from about 10 nanometers to about 100 micrometers,in some embodiments from about 50 nanometers to about 50 micrometers,and in some embodiments, from about 100 nanometers to about 30micrometers. Examples of such fillers include calcium carbonate,silicates, silica, calcium or barium sulfate, aluminum hydroxide, glassfibers or bulbs, wood flour, cellulose powder carbon black, electricallyconductive polymers, etc. The fillers may be introduced into thedispersion in powder form, but may also be present in another form, suchas fibers.

Surface-active substances may also be employed in the dispersion, suchas ionic or non-ionic surfactants. Furthermore, adhesives may beemployed, such as organofunctional silanes or their hydrolysates, forexample 3-glycidoxypropyltrialkoxysilane, 3-aminopropyl-triethoxysilane,3-mercaptopropyl-trimethoxysilane, 3-metacryloxypropyltrimethoxysilane,vinyltrimethoxysilane or octyltriethoxysilane. The dispersion may alsocontain additives that increase conductivity, such as ethergroup-containing compounds (e.g., tetrahydrofuran), lactonegroup-containing compounds (e.g., γ-butyrolactone or γ-valerolactone),amide or lactam group-containing compounds (e.g., caprolactam,N-methylcaprolactam, N,N-dimethylacetamide, N-methylacetamide,N,N-dimethylformamide (DMF), N-methylformamide, N-methylformanilide,N-methylpyrrolidone (NMP), N-octylpyrrolidone, or pyrrolidone), sulfonesand sulfoxides (e.g., sulfolane (tetramethylenesulfone) ordimethylsulfoxide (DMSO)), sugar or sugar derivatives (e.g., saccharose,glucose, fructose, or lactose), sugar alcohols (e.g., sorbitol ormannitol), furan derivatives (e.g., 2-furancarboxylic acid or3-furancarboxylic acid), an alcohols (e.g., ethylene glycol, glycerol,di- or triethylene glycol).

The dispersion may be applied using a variety of known techniques, suchas by spin coating, impregnation, pouring, dropwise application,injection, spraying, doctor blading, brushing, printing (e.g., ink-jet,screen, or pad printing), or dipping. The viscosity of the dispersion istypically from about 0.1 to about 100,000 mPas (measured at a shear rateof 100 s⁻¹), in some embodiments from about 1 to about 10,000 mPas, insome embodiments from about 10 to about 1,500 mPas, and in someembodiments, from about 100 to about 1000 mPas.

In certain embodiments, the solid electrolyte is formed from multiplelayers, one or more of which are formed in the manner described above.If desired, the layers may be free from extrinsically conductivepolymers and thus contain only intrinsically conductive polymers. Forexample, in such embodiments, intrinsically conductive polymers mayconstitute about 50 wt. % or more, in some embodiments about 70 wt. % ormore, and in some embodiments, about 90 wt. % or more (e.g., 100 wt. %)of the solid electrolyte.

In certain embodiments, the solid electrolyte may contain one or more“inner” conductive polymer layers and optionally one or more “outer”conductive polymer layers. The term “inner” in this context refers toone or more layers formed from the same material and that overly thedielectric, whether directly or via another layer (e.g., adhesivelayer). The inner layer(s), for example, typically contain intrinsicallyconductive polymers, either alone or in combination with otherconductive polymers. Through the use of such polymers, the presentinventors have discovered that better contact between the solidelectrolyte and the dielectric can be achieved. In one particularembodiment, the inner layer(s) are generally free of extrinsicallyconductive polymers and thus formed primarily from intrinsicallyconductive polymers. More particularly, intrinsically conductivepolymers may constitute about 50 wt. % or more, in some embodimentsabout 70 wt. % or more, and in some embodiments, about 90 wt. % or more(e.g., 100 wt. %) of the inner layer(s). Of course, this is by no meansrequired and in other embodiments, the inner layer(s) may be formed fromother materials, such as from a dispersion of extrinsically conductivepolymer particles. Regardless, one or multiple inner layers may beemployed. For example, the solid electrolyte typically contains from 2to 30, in some embodiments from 4 to 20, and in some embodiments, fromabout 5 to 15 inner layers.

The solid electrolyte may contain only “inner layers” so that it isessentially formed from the same material, i.e., intrinsicallyconductive polymers. Nevertheless, in other embodiments, the solidelectrolyte may also contain one or more “outer” conductive polymerlayers that are formed from a different material than the inner layer(s)and overly the inner layer(s). For example, in embodiments in which theinner layer(s) are formed primarily from intrinsically conductivepolymers, the outer layer(s) may be formed from a dispersion ofextrinsically conductive polymer particles, such as described above.Alternatively, in embodiments in which the outer layer(s) are formedprimarily from intrinsically conductive polymers, the inner layer(s) maybe formed from a dispersion of extrinsically conductive polymerparticles. Regardless, as noted above, the particles of the dispersionmay have an average size (e.g., diameter) of from about 1 to about 150nanometers, in some embodiments from about 2 to about 50 nanometers, andin some embodiments, from about 5 to about 40 nanometers. In oneparticular embodiment, the outer layer(s) are formed primarily from suchparticles in that they constitute about 50 wt. % or more, in someembodiments about 70 wt. % or more, and in some embodiments, about 90wt. % or more (e.g., 100 wt. %) of a respective outer layer. As noted,one or multiple outer layers may be employed. For example, the solidelectrolyte may contain from 2 to 30, in some embodiments from 4 to 20,and in some embodiments, from about 5 to 15 outer layers, each of whichmay optionally be formed from a dispersion of the extrinsicallyconductive polymer particles.

IV. External Polymer Coating

Although not required, an external polymer coating may also be appliedto the anode that overlies the solid electrolyte. The external polymercoating generally contains one or more layers formed from a dispersionof extrinsically conductive polymer particles, such as described above.The external coating may be able to further penetrate into the edgeregion of the capacitor body to increase the adhesion to the dielectricand result in a more mechanically robust part, which may reduceequivalent series resistance and leakage current. Because it isgenerally intended to improve the degree of edge coverage rather toimpregnate the interior of the anode body, the particles used in theexternal coating typically have a larger size than those employed in anyoptional dispersions of the solid electrolyte. For example, the ratio ofthe average size of the particles employed in the external polymercoating to the average size of the particles employed in any dispersionof the solid electrolyte is typically from about 1.5 to about 30, insome embodiments from about 2 to about 20, and in some embodiments, fromabout 5 to about 15. For example, the particles employed in thedispersion of the external coating may have an average size of fromabout 50 to about 500 nanometers, in some embodiments from about 80 toabout 250 nanometers, and in some embodiments, from about 100 to about200 nanometers.

If desired, a crosslinking agent may also be employed in the externalpolymer coating to enhance the degree of adhesion to the solidelectrolyte. Typically, the crosslinking agent is applied prior toapplication of the dispersion used in the external coating. Suitablecrosslinking agents are described, for instance, in U.S. PatentPublication No. 2007/0064376 to Merker, et al. and include, forinstance, amines (e.g., diamines, triamines, oligomer amines,polyamines, etc.); polyvalent metal cations, such as salts or compoundsof Mg, Al, Ca, Fe, Cr, Mn, Ba, Ti, Co, Ni, Cu, Ru, Ce or Zn, phosphoniumcompounds, sulfonium compounds, etc. Particularly suitable examplesinclude, for instance, 1,4-diaminocyclohexane,1,4-bis(amino-methyl)cyclohexane, ethylenediamine, 1,6-hexanediamine,1,7-heptanediamine, 1,8-octanediamine, 1,9-nonanediamine,1,10-decanediamine, 1,12-dodecanediamine, N,N-dimethylethylenediamine,N,N,N′,N′-tetramethylethylenediamine,N,N,N′,N′-tetramethyl-1,4-butanediamine, etc., as well as mixturesthereof.

The crosslinking agent is typically applied from a solution ordispersion whose pH is from 1 to 10, in some embodiments from 2 to 7, insome embodiments, from 3 to 6, as determined at 25° C. Acidic compoundsmay be employed to help achieve the desired pH level. Examples ofsolvents or dispersants for the crosslinking agent include water ororganic solvents, such as alcohols, ketones, carboxylic esters, etc. Thecrosslinking agent may be applied to the capacitor body by any knownprocess, such as spin-coating, impregnation, casting, dropwiseapplication, spray application, vapor deposition, sputtering,sublimation, knife-coating, painting or printing, for example inkjet,screen or pad printing. Once applied, the crosslinking agent may bedried prior to application of the polymer dispersion. This process maythen be repeated until the desired thickness is achieved. For example,the total thickness of the entire external polymer coating, includingthe crosslinking agent and dispersion layers, may range from about 1 toabout 50 μm, in some embodiments from about 2 to about 40 μm, and insome embodiments, from about 5 to about 20 μm.

V. Other Components of the Capacitor

If desired, the capacitor may also contain other layers as is known inthe art. For example, a protective coating may optionally be formedbetween the dielectric and solid electrolyte, such as one made of arelatively insulative resinous material (natural or synthetic). Suchmaterials may have a specific resistivity of greater than about 10 Ω·cm,in some embodiments greater than about 100, in some embodiments greaterthan about 1,000 Ω·cm, in some embodiments greater than about 1×10⁵Ω·cm, and in some embodiments, greater than about 1×10¹⁰ Ω·cm. Someresinous materials that may be utilized in the present inventioninclude, but are not limited to, polyurethane, polystyrene, esters ofunsaturated or saturated fatty acids (e.g., glycerides), and so forth.For instance, suitable esters of fatty acids include, but are notlimited to, esters of lauric acid, myristic acid, palmitic acid, stearicacid, eleostearic acid, oleic acid, linoleic acid, linolenic acid,aleuritic acid, shellolic acid, and so forth. These esters of fattyacids have been found particularly useful when used in relativelycomplex combinations to form a “drying oil”, which allows the resultingfilm to rapidly polymerize into a stable layer. Such drying oils mayinclude mono-, di-, and/or tri-glycerides, which have a glycerolbackbone with one, two, and three, respectively, fatty acyl residuesthat are esterified. For instance, some suitable drying oils that may beused include, but are not limited to, olive oil, linseed oil, castoroil, tung oil, soybean oil, and shellac. These and other protectivecoating materials are described in more detail U.S. Pat. No. 6,674,635to Fife, et al., which is incorporated herein in its entirety byreference thereto for all purposes.

If desired, the part may also be applied with a carbon layer (e.g.,graphite) and silver layer, respectively. The silver coating may, forinstance, act as a solderable conductor, contact layer, and/or chargecollector for the capacitor and the carbon coating may limit contact ofthe silver coating with the solid electrolyte. Such coatings may coversome or all of the solid electrolyte.

The capacitor may also be provided with terminations, particularly whenemployed in surface mounting applications. For example, the capacitormay contain an anode termination to which the anode lead of thecapacitor element is electrically connected and a cathode termination towhich the cathode of the capacitor element is electrically connected.Any conductive material may be employed to form the terminations, suchas a conductive metal (e.g., copper, nickel, silver, nickel, zinc, tin,palladium, lead, copper, aluminum, molybdenum, titanium, iron,zirconium, magnesium, and alloys thereof). Particularly suitableconductive metals include, for instance, copper, copper alloys (e.g.,copper-zirconium, copper-magnesium, copper-zinc, or copper-iron),nickel, and nickel alloys (e.g., nickel-iron). The thickness of theterminations is generally selected to minimize the thickness of thecapacitor. For instance, the thickness of the terminations may rangefrom about 0.05 to about 1 millimeter, in some embodiments from about0.05 to about 0.5 millimeters, and from about 0.07 to about 0.2millimeters. One exemplary conductive material is a copper-iron alloymetal plate available from Wieland (Germany). If desired, the surface ofthe terminations may be electroplated with nickel, silver, gold, tin,etc. as is known in the art to ensure that the final part is mountableto the circuit board. In one particular embodiment, both surfaces of theterminations are plated with nickel and silver flashes, respectively,while the mounting surface is also plated with a tin solder layer.

Referring to FIG. 1, one embodiment of an electrolytic capacitor 30 isshown that includes an anode termination 62 and a cathode termination 72in electrical connection with a capacitor element 33. The capacitorelement 33 has an upper surface 37, lower surface 39, front surface 36,and rear surface 38, Although it may be in electrical contact with anyof the surfaces of the capacitor element 33, the cathode termination 72in the illustrated embodiment is in electrical contact with the lowersurface 39 and rear surface 38. More specifically, the cathodetermination 72 contains a first component 73 positioned substantiallyperpendicular to a second component 74. The first component 73 is inelectrical contact and generally parallel with the lower surface 39 ofthe capacitor element 33. The second component 74 is in electricalcontact and generally parallel to the rear surface 38 of the capacitorelement 33. Although depicted as being integral, it should be understoodthat these portions may alternatively be separate pieces that areconnected together, either directly or via an additional conductiveelement (e.g., metal).

The anode termination 62 likewise contains a first component 63positioned substantially perpendicular to a second component 64. Thefirst component 63 is in electrical contact and generally parallel withthe lower surface 39 of the capacitor element 33. The second component64 contains a region 51 that carries an anode lead 16. In theillustrated embodiment, the region 51 possesses a “U-shape” for furtherenhancing surface contact and mechanical stability of the lead 16.

The terminations may be connected to the capacitor element using anytechnique known in the art. In one embodiment, for example, a lead framemay be provided that defines the cathode termination 72 and anodetermination 62. To attach the electrolytic capacitor element 33 to thelead frame, a conductive adhesive may initially be applied to a surfaceof the cathode termination 72. The conductive adhesive may include, forinstance, conductive metal particles contained with a resin composition.The metal particles may be silver, copper, gold, platinum, nickel, zinc,bismuth, etc. The resin composition may include a thermoset resin (e.g.,epoxy resin), curing agent (e.g., acid anhydride), and coupling agent(e.g., silane coupling agents). Suitable conductive adhesives may bedescribed in U.S. Patent Application Publication No. 2006/0038304 toOsako, et al. Any of a variety of techniques may be used to apply theconductive adhesive to the cathode termination 72. Printing techniques,for instance, may be employed due to their practical and cost-savingbenefits.

A variety of methods may generally be employed to attach theterminations to the capacitor. In one embodiment, for example, thesecond component 64 of the anode termination 62 and the second component74 of the cathode termination 72 are initially bent upward to theposition shown in FIG. 1. Thereafter, the capacitor element 33 ispositioned on the cathode termination 72 so that its lower surface 39contacts the adhesive and the anode lead 16 is received by the upperU-shaped region 51. If desired, an insulating material (not shown), suchas a plastic pad or tape, may be positioned between the lower surface 39of the capacitor element 33 and the first component 63 of the anodetermination 62 to electrically isolate the anode and cathodeterminations.

The anode lead 16 is then electrically connected to the region 51 usingany technique known in the art, such as mechanical welding, laserwelding, conductive adhesives, etc. For example, the anode lead 16 maybe welded to the anode termination 62 using a laser. Lasers generallycontain resonators that include a laser medium capable of releasingphotons by stimulated emission and an energy source that excites theelements of the laser medium. One type of suitable laser is one in whichthe laser medium consist of an aluminum and yttrium garnet (YAG), dopedwith neodymium (Nd). The excited particles are neodymium ions Nd³⁺. Theenergy source may provide continuous energy to the laser medium to emita continuous laser beam or energy discharges to emit a pulsed laserbeam. Upon electrically connecting the anode lead 16 to the anodetermination 62, the conductive adhesive may then be cured. For example,a heat press may be used to apply heat and pressure to ensure that theelectrolytic capacitor element 33 is adequately adhered to the cathodetermination 72 by the adhesive.

Once the capacitor element is attached, the lead frame is enclosedwithin a resin casing, which may then be filled with silica or any otherknown encapsulating material. The width and length of the case may varydepending on the intended application. Suitable casings may include, forinstance, “A”, “B”, “C”, “D”, “E”, “F”,“G”, “H”, “J”, “K”, “L”, “M”,“N”, “P”, “R”, “S”, “T”, “V”, “W”, “Y”, “X”, or “Z” (AVX Corporation).Regardless of the case size employed, the capacitor element isencapsulated so that at least a portion of the anode and cathodeterminations are exposed for mounting onto a circuit board. As shown inFIG. 1, for instance, the capacitor element 33 is encapsulated in a case28 so that a portion of the anode termination 62 and a portion of thecathode termination 72 are exposed.

The present invention may be better understood by reference to thefollowing examples.

Test Procedures

Equivalent Series Resistance (ESR)

Equivalence series resistance may be measured using a Keithley 3330Precision LCZ meter with Kelvin Leads 2 volt DC bias and a 0.5 volt peakto peak sinusoidal signal. The operating frequency may be 100 kHz andthe temperature may be 23° C.±2° C.

Dissipation Factor

The dissipation factor may be measured using a Keithley 3330 PrecisionLCZ meter with Kelvin Leads with 2.2 volt DC bias and a 0.5 volt peak topeak sinusoidal signal. The operating frequency may be 120 Hz and thetemperature may be 23° C.±2° C.

Capacitance

The capacitance may be measured using a Keithley 3330 Precision LCZmeter with Kelvin Leads with 2.2 volt DC bias and a 0.5 volt peak topeak sinusoidal signal. The operating frequency may be 120 Hz and thetemperature may be 23° C.±2° C. In some cases, the “wet-to-dry”capacitance can be determined. The “dry capacitance” refers to thecapacitance of the part before application of the solid electrolyte,graphite, and silver layers, while the “wet capacitance” refers to thecapacitance of the part after formation of the dielectric, measured in14% nitric acid in reference to 1 mF tantalum cathode with 10 volt DCbias and a 0.5 volt peak to peak sinusoidal signal after 30 seconds ofelectrolyte soaking.

EXAMPLE 1

150,000 μFV/g tantalum powder was used to form anode samples. Each anodesample was embedded with a tantalum wire, sintered at 1290° C., andpressed to a density of 5.6 g/cm³. The resulting pellets had a size of5.20×3.70×0.50 mm. The pellets were anodized to 13.0 volts inwater/phosphoric acid electrolyte with a conductivity of 8.6 mS at atemperature of 85° C. to form the dielectric layer. The pellets wereanodized again to 70 volts in a water/boric acid/disodium tetraboratewith a conductivity of 2.0 mS at a temperature of 30° C. for 25 secondsto form a thicker oxide layer built up on the outside. A conductivepolymer coating was then formed by dipping the anodes into a dispersedpoly(3,4-ethylenedioxythiophene) having a solids content of 1.1% andviscosity of 20 mPa·s (Clevios™ K, Heraeus). Upon coating, the partswere dried at 125° C. for 20 minutes. This process was repeated 10times. Thereafter, the parts were dipped into a dispersedpoly(3,4-ethylenedioxythiophene) having a solids content of 2.0% andviscosity 20 mPa·s (Clevios™ K, Heraeus). Upon coating, the parts weredried at 125° C. for 20 minutes. This process was repeated 3 times.Thereafter, the parts were dipped into a dispersedpoly(3,4-ethylenedioxythiophene) having a solids content of 2% andviscosity 160 mPa·s (Clevios™ K, Heraeus). Upon coating, the parts weredried at 125° C. for 20 minutes. This process was repeated 8 times. Theparts were then dipped into a graphite dispersion and dried. Finally,the parts were dipped into a silver dispersion and dried. Multiple parts(400) of 470 μF/6.3V capacitors were made in this manner andencapsulated in a silica resin.

EXAMPLE 2

Capacitors were formed in the manner described in Example 1, except thata different conductive polymer coating was employed. The conductivepolymer coating was formed by dipping the anodes into a solution ofpoly(4-(2,3-dihydrothieno-[3,4-b][1,4]dioxin-2-ylmethoxy)-1-butane-sulphonicacid having a solids content of 1.9% (Clevios™ K, Heraeus). Uponcoating, the parts were dried at 125° C. for 20 minutes. This processwas not repeated. Thereafter, the parts were dipped into a dispersedpoly(3,4-ethylenedioxythiophene) having a solids content of 1.1% andviscosity 20 mPa·s (Clevios™ K, Heraeus). Upon coating, the parts weredried at 125° C. for 20 minutes. This process was repeated 10 times.Thereafter, the parts were dipped into a dispersedpoly(3,4-ethylenedioxythiophene) having a solids content of 2.0% andviscosity 20 mPa·s (Clevios™ K, Heraeus). Upon coating, the parts weredried at 125° C. for 20 minutes. This process was repeated 3 times.Thereafter, the parts were dipped into a dispersedpoly(3,4-ethylenedioxythiophene) having a solids content of 2% andviscosity 160 mPa·s (Clevios™ K, Heraeus). Upon coating, the parts weredried at 125° C. for 20 minutes. This process was repeated 8 times. Theparts were then dipped into a graphite dispersion and dried. Finally,the parts were dipped into a silver dispersion and dried. Multiple parts(400) of 470 μF/6.3V capacitors were made in this manner andencapsulated in a silica resin.

EXAMPLE 3

Capacitors were formed in the manner described in Example 1, except thata different conductive polymer coating was employed. The conductivepolymer coating was formed by dipping the anodes into a solution ofpoly(4-(2,3-dihydrothieno-[3,4-b][1,4]dioxin-2-ylmethoxy)-1-butane-sulphonicacid having a solids content of 1.9% (Clevios™ K, Heraeus). Uponcoating, the parts were dried at 125° C. for 20 minutes. This processwas repeated 10 times. Thereafter, the parts were dipped into adispersed poly(3,4-ethylenedioxythiophene) having a solids content of 2%and viscosity 20 mPa·s (Clevios™ K, Heraeus). Upon coating, the partswere dried at 125° C. for 20 minutes. This process was repeated 3 times.Thereafter, the parts were dipped into a dispersedpoly(3,4-ethylenedioxythiophene) having a solids content of 2% andviscosity 160 mPa·s (Clevios™ K, Heraeus). Upon coating, the parts weredried at 125° C. for 20 minutes. This process was repeated 8 times. Theparts were then dipped into a graphite dispersion and dried. Finally,the parts were dipped into a silver dispersion and dried. Multiple parts(400) of 470 μF/6.3V capacitors were made in this manner andencapsulated in a silica resin.

The finished capacitors of Examples were then tested for electricalperformance. The median results of capacitance, dry/wet capacitance, Dfand ESR are set forth below in Table 1. The wet capacitance was 536.14μF,

TABLE 1 Electrical Properties Dry Cap Wet-to-Dry Cap. Df ESR [μF] [%][%] [mohms] Example 1 119.88 22.36 4.2 61.0 Example 2 233.11 43.48 5.148.3 Example 3 372.64 69.50 7.7 64.2

These and other modifications and variations of the present inventionmay be practiced by those of ordinary skill in the art, withoutdeparting from the spirit and scope of the present invention. Inaddition, it should be understood that aspects of the variousembodiments may be interchanged both in whole or in part. Furthermore,those of ordinary skill in the art will appreciate that the foregoingdescription is by way of example only, and is not intended to limit theinvention so further described in such appended claims.

What is claimed is:
 1. A solid electrolytic capacitor comprising: ananode that contains a dielectric formed on a sintered porous body,wherein the sintered porous body is formed from a valve metal powderhaving a specific charge of about 100,000 microFarads*Volts per gram ormore; and a solid electrolyte overlying the anode, wherein the solidelectrolyte includes an intrinsically conductive polymer containingrepeating units having the following formula (I):

wherein, R is (CH2)a-O—(CH2)b; a is from 0 to 10; b is from 1 to 18; Zis SO₃; and X is an alkali metal; wherein the solid electrolyte isformed from multiple layers, at least one of which contains theintrinsically conductive polymer; wherein the solid electrolyte containsan inner layer and an outer layer, the inner layer containing theintrinsically conductive polymer; wherein the inner layer is free ofextrinsically conductive polymers.
 2. The solid electrolytic capacitorof claim 1, wherein the powder contains primary particles having amedian size of from about 5 to about 500 nanometers.
 3. The solidelectrolytic capacitor of claim 1, wherein the powder includes tantalum.4. The solid electrolytic capacitor of claim 1, wherein a is 1 and b is3 or
 4. 5. The solid electrolytic capacitor of claim 1, wherein thepolymer is a homopolymer.
 6. The solid electrolytic capacitor of claim5, wherein the homopolymer ispoly(4-(2,3-dihydrothieno-[3,4-b][1,4]dioxin-2-ylmethoxy)-1-butane-sulphonicacid, salt),poly(4-(2,3-dihydrothieno-[3,4-b][1,4]dioxin-2-ylmethoxy)-1-propanesulphonicacid, salt), or a combination thereof.
 7. The solid electrolyticcapacitor of claim 1, wherein the solid electrolyte further includes anextrinsically conductive polymer having repeating units of the followingformula (III):

wherein, R₇ is a linear or branched, C₁ to C₁₈ alkyl radical; C₅ to C₁₂cycloalkyl radical; C₆ to C₁₄ aryl radical; C₇ to C₁₈ aralkyl radical;or C₁ to C₄ hydroxyalkyl radical, or hydroxyl radical; and q is aninteger from 0 to
 8. 8. The solid electrolytic capacitor of claim 7,wherein the extrinsically conductive polymer ispoly(3,4-ethylenedioxthiophene).
 9. The solid electrolytic capacitor ofclaim 1, wherein each of the layers contains the intrinsicallyconductive polymer.
 10. The solid electrolytic capacitor of claim 1,wherein the outer layer contains an extrinsically conductive polymer.11. The solid electrolytic capacitor of claim 10, wherein theextrinsically conductive polymer is poly(3,4-ethylenedioxthiophene). 12.The solid electrolytic capacitor of claim 11, wherein the outer layerfurther comprises a polymeric counter anion.
 13. The solid electrolyticcapacitor of claim 1, further comprising an external polymer coatingthat overlies the solid electrolyte, wherein the external polymercoating contains conductive polymer particles.
 14. The solidelectrolytic capacitor of claim 1, further comprising an anodetermination that is electrically connected to the anode and a cathodetermination that is electrically connected to the solid electrolyte. 15.The solid electrolytic capacitor of claim 1, wherein the capacitorexhibits a wet-to-dry capacitance of about 50% or more.
 16. A solidelectrolytic capacitor comprising: an anode that contains a dielectricformed on a sintered porous body, wherein the sintered porous body isformed from a valve metal powder having a specific charge of about100,000 microFarads*Volts per gram or more; and a solid electrolyteoverlying the anode, wherein the solid electrolyte includes anintrinsically conductive polymer containing repeating units having thefollowing formula (I):

wherein, R is (CH2)a-O—(CH2)b; a is from 0 to 10; b is from 1 to 18; Zis SO₃; and X is an alkali metal; wherein the solid electrolyte is freeof extrinsically conductive polymers.
 17. The solid electrolyticcapacitor of claim 16, further comprising an external polymer coatingthat overlies the solid electrolyte, wherein the external polymercoating contains conductive polymer particles.
 18. A method for forminga solid electrolytic capacitor, the method comprising: pressing a powderinto the form of a pellet, wherein the powder has a specific charge ofabout 100,000 pF*V/g or more; sintering the pellet; anodically oxidizingthe sintered pellet to form a dielectric layer that overlies the anode;and dipping the anodically oxidized pellet into a solution that containsan intrinsically conductive polymer containing repeating units havingthe following formula (I):

wherein, R is (CH2)a-O—(CH2)b; a is from 0 to 10; b is from 1 to 18; Zis SO₃; and X is an alkali metal; wherein the solid electrolyte isformed from multiple layers, at least one of which contains theintrinsically conductive polymer; wherein the solid electrolyte containsan inner layer and an outer layer, the inner layer containing theintrinsically conductive polymer; wherein the inner layer is free ofextrinsically conductive polymers.
 19. The method of claim 18, whereinthe polymer ispoly(4-(2,3-dihydrothieno-[3,4-b][1,4]dioxin-2-ylmethoxy)-1-butane-sulphonicacid, salt),poly(4-(2,3-dihydrothieno-[3,4-b][1,4]dioxin-2-ylmethoxy)-1-propanesulphonicacid, salt), or a combination thereof.
 20. The method of claim 18,further comprising applying a dispersion to the anodically oxidizedpellet after dipping into the solution, wherein the dispersion containsextrinsically conductive polymer particles.
 21. The method of claim 20,wherein particles contain poly(3,4-ethylenedioxthiophene) andpolystyrene sulfonic acid.